Polymers and copolymers of isobutylene are well known in the art. In particular, copolymers of isobutylene with conjugated multiolefins have found wide acceptance in the rubber field. These polymers are generally termed in the art "butyl rubber". The preparation of butyl rubber is described in U.S. Pat. No. 2,356,128, which is incorporated herein by reference.
The term "butyl rubber" as employed in the specification is intended to include copolymers made from the polymerization of a reaction mixture comprising an isoolefin having about 4 to 7 carbon atoms, e.g. isobutylene and a conjugated multiolefin having about 4 to 14 carbon atoms, e.g. isoprene. Although these copolymers are said to contain about 0.2 to about 15% combined multiolefin, in practice the butyl rubber polymers of commerce contain about 0.6 to about 4.5 wt. % of multiolefin; more generally, about 1.0 to about 1.8 wt. %, the remainder of the polymer being comprised of the isoolefin component.
Efforts to prepare isoolefin-multiolefin polymers of high unsaturation have met with varying degrees of success. Where substantially gel-free polymers have been prepared containing more than about 5% multiolefin, the polymers have been of low number average molecular weight. This has been true even where these polymers had high viscosity average molecular weights. In general, however, the products formed by prior art processes are either high in gel content or low in number average molecular weight and of little utility.
Multiolefins are known to be molecular weight and catalyst poisons; furthermore, increased unsaturation in the polymer backbone provides potential sites for gelation. Hence, attempts to prepare more highly unsaturated isoolefinmultiolefin copolymers by prior art methods have resulted in the formation of either low molecular weight or resinous crosslinked polymers which have little or no commercial utility as elastomers.
Although some commercial elastomers such as styrene butadiene rubber or EPDM may contain as much as 2 to 9% gel, isobutylene copolymers of commerce are substantially gel free. The isobutylene copolymers may contain as much as 2% gel but preferably contain less than 1%.
There are numerous patents and literature disclosures which generally disclose polymers and copolymers of isobutylene, the copolymers purportedly having from about 0.5 to 98% unsaturation. Where the prior art copolymers are high in unsaturation, however, they are either low in number average molecular weight or resinous.
Japanese Pat. No. JA27416/68 published Nov. 26, 1968 teaches a process for preparing copolymers of conjugated diene compounds with isobutylene which contain "a large amount of conjugated diene compounds" using catalysts prepared by reacting (1) mercuric halide, aluminum halide or hydrogen halide, (2) zirconium halide and (3) aluminum metal in the presence of an aromatic compound, e.g. benzene. These products are described as copolymers which are "rubbery substances when the isobutylene is high and are resinous when the isobutylene content is low". The resinous properties result from gelation and crosslinking of the polymer during its preparation. These gelled and crosslinked products have little utility as rubbers. The products of lower unsaturation, i.e. high isobutylene content rubbers, are of the conventional butyl rubber type.
Japanese Patent JA27417/68 published Nov. 26, 1968 teaches a method for preparing copolymers of dienes and isoolefins containing about 0.1 to about 40 wt. %, preferably about 0.5 to 5 wt. % of diene. The polymers are prepared using a catalyst derived from (1) metal oxides of the general formula M.sub.x O.sub.y, wherein M is nickel or cobalt and 1&lt;y/x.ltoreq.1.5, and (2) aluminum halide. Again, the low unsaturation polymers are the conventional butyl rubbers whereas the highly unsaturated materials are either low in number average molecular weight or are gelled polymers.
U.S. Pat. No. 3,466,268 and its parent counterpart, U.S. Pat. No. 3,357,960 disclose a butadiene isobutylene copolymer and a process for preparing said copolymer. The invention disclosed is a method of improving butadiene polymers by incorporating in the structure varying amounts of isobutylene. Preferably, the amount of isobutylene incorporated is said to be about 2 to 40 wt. %. The polymers disclosed are generally low in number average molecular weight. Substitution of isoprene for butadiene results in highly crosslinked copolymers which have little utility.
U.S. Pat. No. 2,772,255 (Br. 744,514) discloses a method for preparing high molecular weight butyl rubbers. In general, the polymers which are prepared are conventional butyl rubbers having less than 3 mole % unsaturation. Attempts to produce butyl rubber type polymers having unsaturation in excess of 5 mole % unsaturation result in products which either are low in number average molecular weight or are gelled and highly crosslinked.
High unsaturation isobutylene-isoprene copolymers have been prepared (see, for example, U.S. Pat. No. 3,242,147 incorporated herein by reference). Although these polymers are purportedly high in viscosity average molecular weight, the number average molecular weights are low. Hence, the products have little commercial significance.
U.S. Pat. No. 2,521,359 to Garber discloses copolymers consisting primarily of cyclopentadiene with minor amounts of other monomers (including isobutylene) as resins capable of air curing to tough coatings and films. The copolymers of Garber having more than 50% cyclopentadiene are of extremely low molecular weight.
Unlike plastics, elastomers require a high number average molecular weight in order to realize desirable levels in physical properties. For example, tensile strength for elastomers is critically dependent on number average molecular weight since these polymers are used well above their glass transition temperature and are generally amorphous.
In contrast to elastomers, plastics are used well below their glass transition temperature and it is molecular associations which gives them their structural integrity. As a result, number average molecular weights in the order of 10,000 to 70,000 are adequate for commercial utility.
Elastomers, on the other hand, obtain their structural integrity from a crosslinked network. Perfection of this network is directly dependent on the length of the polymer molecules from which the network is derived. Number average molecular weight (Mn) is a measure of the length of the molecules. Viscosity or weight average molecular weights are misleading measurements since their numerical value is greatly affected by small variations in the distribution of the higher molecular weight fractions. Hence, polymers of low number average molecular weight may have high viscosity average molecular weight as a result of disproportionate distribution of the high molecular weight fraction.
The importance of number average molecular weight on tensile strength has long been recognized (see, for example, Flory, p. 5, Ind. Eng. Chem., 38, 417 (1946), incorporated herein by reference. Flory showed that, for low unsaturation elastomeric copolymers of isobutylene, tensile strength increased rapidly as the number average molecular weight was increased beyond a minimum value (i.e. 100,000) then approaches an asymptotic limit.
For economic reasons, oil extendability is an essential characteristic of a commercial elastomer for almost all major uses. The tensile strength of butyl rubber vulcanizates is reduced by the addition of oil, and to retain the original tensile strength of the undiluted composition it is necessary to increase the number average molecular weight. Oil extension also improves the low temperature properties of butyl inner tubes and when this phenomenon was discovered, it was necessary to develop higher molecular weight polymers to accommodate the added oil. See, for example, Buckley et al, Ind. Eng. Chem., 42, 2407 (1950).
This finding resulted in the rapid adoption by industry of the high molecular weight type of butyl GR-1-18 with Mooney viscosity greater than 71 (212.degree. F.). These materials generally have number average molecular weights of 150,000 or greater. In contrast, the previously used polymers which have number average molecular weights of less than 120,000 with Mooney viscosity specification of 38-49 (212.degree. F.) were limited to applications which did not require oil extension, and today represent a very minor portion of the butyl rubber market having been supplanted almost entirely by the higher molecular weight butyl rubbers.
Although it has been postulated that higher unsaturation copolymers of isobutylene would be attractive polymers, useful polymers have not been available since the prior art methods are not capable of producing highly unsaturated, e.g., at least 5 mole % to about 45 mole %, isobutylene copolymers of sufficiently high number average molecular weight. Hence, the prior art isobutylene-conjugated diene copolymers offered commercially are low in unsaturation, e.g., 1-4.5 mole %.
Hence, heretofore, methods of preparing copolymers of isoolefins and conjugated dienes have not included a means for making commercial quality elastomers containing greater than 5 mole % diene.
Although the isobutylene-conjugated dienes of commerce have improved ozone resistance, these polymers are still subject to ozone cleavage since the site of unsaturation is in the polymer backbone. It has been postulated that isobutylene copolymers having unsaturation on the side chain rather than the backbone would be highly resistant to ozone attack. Attempts to produce such polymers using cyclopentadiene as the diene comonomer have been notably unsuccessful.
Isobutylene-cyclopentadiene copolymers and terpolymers of the prior art have been too low in molecular weight to be of commercial significance. Some improvement in molecular weight has been accomplished by copolymerizing isobutylene with minor amounts of cyclopentadiene (CPD) along with other monomers including crosslinking agents such as divinyl benzene. The resulting products are somewhat improved terpolymers or tetrapolymers resulting from the linking of the low molecular weight isobutylene-CPD chains into two dimensional highly branched polymers. Such polymers, however, have inferior physical properties as compared to the butyl rubbers of commerce and hence have not gained acceptance.
A review of the art illustrates the problems encountered where attempts were made to prepare copolymers of isobutylene and cyclopentadiene (CPD). For example, U.S. Pat. No. 2,577,822, incorporated herein by reference, teaches the need for the addition of divinyl benzene in order to compensate for the deleterious effect of CPD on molecular weight.
U.S. Pat. No. 3,080,337, incorporated herein by reference, teaches the addition of isoprene as a third monomer but the resulting products are low in unsaturation and have poor physical properties. Others have made various attempts to produce CPD isoolefin copolymers with varying degrees of success; see, for example, U.S. Pat. Nos. 3,239,495; 3,242,147; 2,521,359; British Pat. No. 1,036,618 and I & EC Prod R and D, 1, 216-20 (1962) incorporated herein by reference. These polymers, however, have substantially no commercial significance because, even when only minor amounts of CPD were present, they are low in number average molecular weight.